The present invention relates to a method for driving a display element comprising an organic light-emitting element that emits light by passing a current through an organic thin film.
In recent years, the research of organic light-emitting elements has been activated due to the low drive voltage required to emit light and the possibility of selecting light-emitting colors through the application of various light-emitting materials (for example, U.S. Pat. No. 3,530,325). Such research has been accelerated since it was reported that a laminated organic light-emitting element formed of an anode/positive hole injection layer/light-emitting layer/cathode is used to achieve a brightness of 1,000 cd/m.sup.2 or higher at a drive voltage of 10 V or lower (U.S. Pat. No. 4,356,429).
In addition, in recent years, active attempts have been made to form a display panel by arranging elements in a matrix (for example, Japanese Patent Application Laid Open No. 2-66873 corresponding to U.S. patent application Ser. No. 211,616 filed on Jun. 27, 1988, and Japanese Pat. Application Laid Open No. 2-148687).
A major problem of an organic light-emitting element is that the light-emission brightness decreases with increasing a period of lighted time. In particular, the brightness decreases rapidly if the voltage applied to the element is constant. This is primarily because the internal resistance of the element increases with an increase of a period of a lighted time, thereby reducing the amount of current. This in turn causes the sticking of a display panel comprising the organic light-emitting elements.
To prevent such sticking, a pixel of the organic light-emitting element may be driven at a constant current. This may mitigate the speed at which the brightness decreases. The constant current drive method, however, does not eliminate the cause of the increase in the internal resistance of the element, and changes still occur inside the element that prevents the light emission.
Thus, an object of this invention is to provide a method of driving an organic light-emitting element-display element that reduces a possibility of sticking, that can prevent brightness from being reduced, and that has a long life expectancy.